1. Field of the Invention
The present invention relates to determination of a location of user equipment in a third generation wideband code division multiple access network.
2. Description of the Related Art
Third Generation (3G) mobile communication systems include LoCation Services (LCSs) which measure radio signals to determine the geographic location of a User Equipment (UE). The location information may be requested by and reported to a client and/or application associated with the UE or an external client in the Core Network (CN) of the 3G mobile communication system. Furthermore, the location information is also used by the Universal Terrestrial Radio Access Network (UTRAN) of the 3G mobile communication system to facilitate location assisted handovers or to support other features, such as home location billing.
The LCSs for 3G mobile communication systems are based on methods that have been used for the Global System for Mobile Communication (GSM) which include Time of Arrival (TOA), Observed Time Difference of Arrival (OTDOA), and Global Positioning System (GPS). These positioning methods are described in technical specification GSM 03.71, version 7.2.1. The TOA method comprises transmitting a signal from a UE and measuring the time of arrival of the signal at three or more measurement units. The difference in times of arrival is determined by pair-wise subtraction of the measured times of arrival. The mobile position may then be calculated via hyperbolic trilateration. However, to determine the actual time differences, the real time difference (RTD) between the three measurement units must be known or determined.
The OTDOA method of LCS measures the difference in time of arrival at the UE of the signals from several nodes or Base Station Transceivers (BTSs). This method also requires that the RTD between the BTSs be determined. The determination of the RTD for both the TOA and OTDOA methods of LCS is complex and therefore reduces the efficiency of the system.
However, the GPS assisted method of location requires that the UE have a GPS receiver. This requirement thus adds both bulk and cost to the UE.
Another method for determining a geographic location of the UE in a 3G wireless network includes measuring a round trip time (RTT) between the UE and at least three radio transmission nodes of the wireless network. The RTT is the time required for a downlink (DL) transmission from a node to a UE to the reception of an uplink (UL) transmission at the node from the UE in response to the DL transmission. FIG. 8 illustrates the RTT. At time t1 a node-B of a wireless network begins transmission of a DL transmission in a dedicated physical channel (DPCH) path. The node-B is a logical node responsible for radio transmission and/or reception in one or more cells with the UE and is similar in function to a Base Transceiver Station (BTS) in GSM. It will be generically referred to as a transceiver node in the following discussion. The DL transmission is received by the UE at time t2, which is a one-way propagation delay Tp after the time t1. After the passage of time equal to a nominal transmission timing delay T0, the UE begins transmission of a UL transmission in the DPCH path at time t3 in response to the DL transmission. After another one-way propagation delay Tp, the transceiver node receives the UL transmission at time t4. Therefore, the RTT comprises a first one-way propagation delay, the UE reception to transmission time delay (UE Rx-Tx) (also referred to as a transmission timing delay), and a second one-way propagation delay or RTT=Tp+T0+Tp. The nominal value of the transmission timing delay T0 is defined as a constant value of 1024 chips. A chip is the longest duration of a spread spectrum signal in which signal parameters are approximately constant, i.e. the duration of time of the most elemental component of a spread spectrum signal when it is decompressed in time.
An RTT measured by a transceiver node in a 3G mobile communication system is roughly defined by the time difference between t1 and t4. Since the nominal value of the transmission timing delay or DL-UL timing offset T0 at the UE (t3-t2) is known, the round trip propagation delay (RTPD), which equals 2Tp can be calculated by subtracting To from the RTT. Since RTPD is related to the distance (i.e., time x velocity), the location of the UE can be estimated if the UE is connected to three or more nodes of the UTRAN whose locations are known.
However, in a 3G network, the transmission timing difference, which is the time between reception of the DL transmission at the UE and transmission of the UL transmission from the UE (i.e., t3-t2), is not a fixed time period and may be different from the nominal transmission timing delay T0 when (1) the UE is moving relatively fast (i.e., an automobile at highway speed) toward or away from the BTS, (2) the propagation paths vary, and (3) there is, or has been, a soft handover of the UE from one cell to another. Accordingly, a geographic location based on the RTT using the nominal transmission timing delay T0 can be significantly inaccurate (the maximum inaccuracy in a location is typically greater than a cell radius).
The inaccuracy in determining the propagation delay associated with the RTT measurements caused by the difference between the actual transmission timing delay and the nominal transmission timing delay To may be corrected by separately measuring the RTT for each cell in active communication with the UE. This prior art method eliminates the requirement for determining the real time difference (RTD) between the various nodes used for measurement. However, it requires more than one transceiver node in active communication with the UE, i.e., connected to the UE.